feat: Plan Canvas, a browser review canvas for plans (#2467)

* feat: add Plan Canvas - browser annotate-and-approve review for plan artifacts

- scripts/plan-canvas.js CLI (open/await/end/stop/server; bin ecc-plan-canvas)
- loopback server + ECC-styled chrome + annotation SDK + zero-dep markdown renderer
- Approve/Request-changes verdicts wired to the /plan confirmation gate
- plan-canvas skill, /plan-canvas command, SessionStart hook surfacing open reviews
- shared scripts/lib/loopback-guard.js extracted from control-pane (API re-exported)
- 121 new tests incl. full-workflow E2E; registered in manifests, catalog, registry

Inspired by lavish-axi (https://github.com/kunchenguid/lavish-axi) by @kunchenguid;
original ECC-native implementation, not a port.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* refactor(plan-canvas): invoke via ecc-plan-canvas bin so the skill works from any project

Skill/command referenced a cwd-relative `node scripts/plan-canvas.js`, unusable
outside the ECC root. Switch to the ecc-plan-canvas bin (and $CLAUDE_PLUGIN_ROOT
fallback) and align CLI next_step hints so an agent can run it as a skill in any repo.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* feat(plan-canvas): render Mermaid diagrams + ship Codex cross-harness surface

- markdown renderer emits <pre class="mermaid"> for ```mermaid blocks (source
  entity-escaped so the browser decodes it for the renderer while blocking injection)
- artifact template loads a pinned Mermaid build only when a diagram is present,
  themed to ECC dark, securityLevel strict, graceful offline fallback to source
  (ECC_PLAN_CANVAS_MERMAID_URL overrides for a local mirror)
- skill teaches Mermaid-for-diagrams and states the CLI+JSON loop is harness-agnostic
- add .agents/skills/plan-canvas (Codex) with agents/openai.yaml interface manifest
- register in install-modules workflow-quality paths; docs updated

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* docs(plan-canvas): add demo screenshot

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(ci): sync yarn.lock with new bin; add contributor checklist

- yarn.lock records the ecc-plan-canvas bin so Yarn hardened-mode install no
  longer wants to modify the lockfile on public PRs
- PR template + CONTRIBUTING gain a pre-push checklist covering the lockfile
  trap and the full skill/command/CLI registration surfaces

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Haley Chen <2022hachen@gmail.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

CONTRIBUTING.md

@@ -464,7 +464,19 @@ How you tested this.
 - [ ] Clear descriptions
 ```
 
-### 3. Review Process
+### 3. Before You Push (avoid red CI)
+
+Run `npm test` locally. It is the same gauntlet CI runs, and it catches almost everything below.
+
+- **Changed `package.json`?** If you touched `bin`, `files`, or dependencies, run `yarn install --mode=update-lockfile` and commit the `yarn.lock` change. CI runs Yarn in hardened mode on public PRs and fails if the lockfile would be modified, so a stale `yarn.lock` breaks the build on its own.
+- **Added a skill, command, agent, hook, or CLI tool?** Wire up every surface it belongs to:
+  - `package.json` (`bin` and `files`), `manifests/install-components.json`, `manifests/install-modules.json`, and `agent.yaml`
+  - Regenerate the catalog (`npm run catalog:sync`) and command registry (`npm run command-registry:write`)
+  - Update the docs tables (`README.md`, `COMMANDS-QUICK-REF.md`, `docs/COMMAND-AGENT-MAP.md`)
+  - New script path? Add it to the publish surface allowlist (`tests/scripts/npm-publish-surface.test.js`)
+  - Cross-harness: for Codex, add `.agents/skills/<name>/` plus `agents/openai.yaml`. The Codex frontmatter validator only allows `name`, `description`, `metadata`, `license`, and `allowed-tools`, so drop keys like `version` from that copy.
+
+### 4. Review Process
 
 1. Maintainers review within 48 hours
 2. Address feedback if requested